Many of the manufacturing steps used to make semiconductor devices and integrated circuits require that a very high purity gaseous atmosphere be maintained within the vessel used for processing. Solid particles and vapor phase contaminants, notably moisture and oxygen, must be kept to an absolute minimum. Since these contaminants are contained in air, air infiltration into the vessel must be minimized.
Among the processes strongly affected by these contaminants is the annealing of titanium-coated silicon wafers in a nitrogen or argon environment to promote the formation of titanium silicide. As little as 1 to 5 ppm of oxygen and moisture can lead to the formation of undesirable titanium oxide. Similarly, the deposition of polysilicon onto exposed silicon on wafers to form bipolar emitter structures is very sensitive to oxidation of the exposed silicon. If air is initially present in the low pressure processing vessel, an oxide will form on the exposed silicon surfaces which can degrade device performance.
The extent of air infiltration into process vessels, the forces governing this phenomenon and the problems it can cause are not fully recognized in the integrated circuit industry. Most commercial semiconductor equipment is operated in batch or semibatch fashion. A load of one or more wafers is placed into a vessel, processed and unloaded. Measurements of the purity of the atmosphere within a wide variety of processing vessels during all phaess of their operation reveal that typically considerable infiltration of room air into a processing vessel occurs during loading and unloading, which leads to detrimental contamination.
A processing vessel usually must be maintained at elevated temperature at all times because of the long times required for cooling and heating. The primary force for the influx of air into the vessel is the buoyancy difference between cool room air and the hot gases in the vessel. Hot gases in the vessel tend to rise toward the top of the vessel; cool room air flows in and down toward the bottom of the vessel through any openings in the vessel. Thus when a vessel is opened to admit or remove a workpiece, a large influx of room air occurs, and the atmosphere in the vessel becomes seriously contaminated.
In commercial practice, wafers are often placed in a boat on an open, motorized cantilevered carrier. When the processing vessel is opened, the carrier is transported into the processing vessel. As the wafers approach the processing vessel, the wafers are heated by radiant and convective heat transfer from the processing vessel. At this point, the wafers are still in a room air environment which is often deleterious to them as their temperature rises. Then, as wafers enter the processing vessel, they are exposed to still higher temperatures and to large concentrations of oxygen, moisture and particles which entered when the processing vessel was open.
Once the vessel is closed, the process gases begin to purge the airborne contaminants from the vessel. However, purging to levels below 100 ppm can take tens of minutes. As a result, process time is lost, equipment throughput decreases and capital costs per wafer processed increase.
The purging problem is even more severe for a low pressure or evacuated vessel. Here, moisture from infiltrating air adsorbs on the inner surface of the vessel and all other exposed components. Its removal under vacuum is difficult and slow.